1. Field of the Invention
The present invention relates to a BAW (bulk acoustic wave) apparatus, such as a BAW filter.
2. Description of Prior Art
Due to their high resonance frequencies and their high quality, in particular in mobile radio communication, BAW resonators have been widely used. The simplest implementation of a BAW resonator comprises a thin layer of piezoelectric material arranged between two metal electrodes. Common piezoelectric materials are, for example, aluminum nitride (AlN) or zinc oxide (ZnO). FIG. 3 shows an exemplary BAW resonator 30 having a static capacitance C which comprises a layer of piezoelectric material which will be referred to as piezo layer 32 below and is located between a first electrode, or top electrode T, and a second electrode, or bottom electrode B. The designations top electrode and bottom electrode merely serve definition purposes and do not represent any limitation with regard to the spatial arrangement and positioning of the BAW resonator. Rather, the designations top electrode and bottom electrode serve to define the positions of these electrodes in relation to a polarization of the piezoelectric material, as will be explained below, so that the polarization of the respective BAW resonators can be derived from an equivalent circuit diagram designating the T and B electrodes.
If an electric field is applied between first electrode T and second electrode B of BAW resonator 30, the reciprocal or inverse piezoelectric effect will cause BAW resonator 30 to mechanically expand or contract, the case of expansion or of contraction depending on the polarization of the piezoelectric material, as has been mentioned above. This means that the opposite case applies if the electric field is inversely applied between the T and B electrodes. In the case of an alternating field, an acoustic wave is generated in piezo layer 32, and, depending on the implementation of the BAW resonator, this wave will propagate, for example, in parallel with the electric field, as a longitudinal wave, or, as a transversal wave, transverse to the electric field, and will be reflected, for example, at the interface of piezo layer 32. Whenever the thickness d of piezo layer 32 equals an integer multiple of half the wavelength λ of the acoustic waves, resonance states and/or acoustic resonance vibrations will occur. The fundamental resonance frequency, i.e. the lowest resonance frequency fres, will then be inversely proportional to thickness d of piezo layer 32. This means that the BAW resonator vibrates at the frequency specified externally.
The piezoelectric properties and, thus, also the resonance properties of a BAW resonator depend on various factors, e.g. on the piezoelectric material, the production method, the polarization impressed upon the BAW resonator during manufacturing, and the size of the crystals. As has been mentioned above, it is particularly the resonance frequency which depends on thickness d of the piezo layer.
As has been mentioned above, BAW resonators exhibit electric polarization. The direction of mechanical deformation, expansion or contraction, of the BAW resonator depends on the direction of the electric field applied to first electrode T and second electrode B and on the direction of polarization of BAW resonator 30. For example, if the polarization of the BAW resonator and the direction of the electric field are pointing in the same direction, BAW resonator 30 contracts, whereas BAW resonator 30 expands when the polarization of BAW resonator 30 and the direction of the electric field are pointing in the opposite direction.
BAW resonators have been known in various configurations. Typically, one differentiates between so-called FBARs (film bulk acoustic resonators) and SMRs (solidly mounted resonators). In addition, technology has known not only of BAW resonators having one piezo layer 32, but also of BAW resonators having several piezo layers.
BAW resonators are employed for filters, for example. FIG. 4 shows a circuit diagram of a possible BAW filter in a ladder configuration, comprising electric ground 42, a signal input 44, a signal output 46, a first series resonator 48, a second series resonator 50, a third series resonator 52, a first shunt resonator 54, a second shunt resonator 56, a third shunt resonator 58 and a fourth shunt resonator 60, the series resonators 48, 50, 52 being connected in series between signal input 44 and signal output 46, and the first shunt resonator 54 being connected in parallel between signal input 44 and electrical ground 42, the second shunt resonator being connected between a connection node between first series resonator 48 and second series resonator 50 and electrical ground 42, third shunt resonator 58 being connected between a connection node between second series resonator 50 and third series resonator 52 and the electrical ground, and fourth shunt resonator 60 being connected in parallel between signal output 46 and electrical ground 42. Each of the series and shunt resonators comprises a top electrode T and a bottom electrode B, which are indicated in the equivalent circuit diagram of FIG. 4 so as to indicate the polarization of the BAW resonators.
A problem with, e.g., filter circuits is the thermal stress of the BAW resonators. However, this problem may be solved, for example, by the method of “cascading” of BAW resonators, which is widely utilized with BAW devices. In the following, cascade is to mean a chain, or series connection of elements. A BAW resonator exhibiting static capacitance C is replaced by a cascade of two BAW resonators, each exhibiting a static capacitance 2C, so that the total capacitance is again C. In principle, such a cascaded BAW resonator has the same impedance properties as a corresponding individual BAW resonator. The main motivation for cascading is, as has been mentioned above, to reduce thermal stress of BAW resonators by a factor of 4, the reduction of the thermal stress resulting from the fact that a cascaded pair of BAW resonators exhibiting static capacitance 2C is larger, by a factor of 4, than a corresponding individual BAW resonator exhibiting static capacitance C.
US 005231327 A describes, for example, such an approach for cascading, wherein a BAW resonator exhibiting static capacitance C is replaced by a series connection of two BAW resonators exhibiting static capacitance 2C, these two BAW resonators sharing one piezo layer.
Another problem is that BAW devices, e.g. BAW resonators, BAW filters, or BAW antenna duplexers, which are operated directly at the antenna of a mobile radio system, exhibit a non-linear behavior. This problem occurs, for example, with BAW antenna duplexers if the transmitting power levels exceed 0.1 W.
A side effect of the above-mentioned cascading is that the energy density is also smaller by a factor of 4, and that, thus, non-linear effects are reduced by 6 dB with a cascaded resonator.
However, a major disadvantage of cascading is that cascading considerably increases the size of the BAW device if it is carried out for all BAW resonators of a filter. It is generally impossible to utilize cascading for all resonator branches of a filter; usually cascading is applied only to the smallest BAW resonators in the direct signal path, e.g. to series resonators 48, 50, 52 in a filter having a ladder configuration in accordance with FIG. 4, since these BAW resonators are most affect by the risk of overheating.